IGBT (insulated gate bipolar transistor) chips are conventionally designed with an array of homogenous IGBT cells, so that the IGBT has a uniform characteristic. Such a homogenous design ideally results in homogeneous current distribution within the device. In reality however, a small, unintended non-homogeneity in the construction of the IGBT cells and corresponding current unbalance is unavoidable due to process variations. A slight, unavoidable non-homogeneity between and within the cells of the IGBT can lead to current filaments (spikes) during switching of the device. These current filaments can cause the local current density to exceed 10× the average current value and enhance transient avalanche dramatically, especially in IGBTs for high voltage and high current applications. Transient avalanche in IGBTs can become so strong that transient avalanche oscillation (TAO) occurs at frequencies of up to hundreds of MHz (even GHz), causing EMI disturbances.
Impact ionization can occur if the terminal voltage of a p-n diode rises above the breakdown voltage of the diode and the maximum electric field overrides the critical value. The generated electrons and holes drift through the depletion region and influence current at the terminal electrodes. The time needed by the charge generation and charge transmission through the depletion region leads to phase shift between the terminal voltage and the influenced current. The diode works as an oscillator, if the phase shift lies around 180°. If the power diode is supplied with a DC-link voltage a short time after the zero current crossing point at a low working temperature range (approximately T<280K), a dynamic IMPATT (impact ionization and transit time) oscillation can occur. This dynamic IMPATT oscillation is caused by the K-center, which is created at irradiation of the semiconductor with high energy particles and leads to temporary reduced blocking voltage.
In the turn off process of IGBTs, a portion of plasma can remain near the anode side of the device, after the IGBT has taken over the blocking voltage, the depletion region and the electrical field is built up. This residual plasma is finally removed by the tail current. The excitation mechanism based on periodic hole-current extraction from the electron-hole plasma can cause the IGBT chip to oscillate with a frequency up to several hundred MHz. Under certain conditions, random high frequency oscillations can occur during the tail phase of IGBTs which are paralleled in power modules. Such oscillations can be suppressed by suitable layout modifications to mismatch the Eigen-frequency of the module setup and the PETT (plasma transit time mechanism) frequency.
In modern IGBTs high plasma density and high storage charge are generated to enable a low forward voltage during the on-state. In the on-state, the proportionality between electron current and hole current is about 3 (i.e. the ratio of electron mobility to hole mobility). In a typical turn-off process, after the so-called Miller plateau, the IGBT is in the active state while its MOSFET part operates in the saturation region. A further drop in the gate voltage leads to further reduction of channel current (electron current). This changes the proportion between electron and hole current. At the moment of the MOS channel current turn-off, the total or a large part of the load current still flows. As such, most of the current must be carried by the holes instead of electrons. At this moment, the net hole-density near the emitter side raises according to the current share of holes as the electron-density reduces. This results in a net hole distribution (hole-density minus electron-density) near the MOS channel and allows the electrical field gradient (given by Poisson's equation) and thus the electric field to build-up near the emitter area.
An abrupt turn-off of the MOS channel current leads to a high transient electrical field peak near the emitter side, which exceeds the critical field strength of silicon and leads to avalanche. The electrons generated by the avalanche drift through the space charge area, also referred to as depletion region, to the anode side under the influence of a strong electric field. Similar to the IMPATT mechanisms previously described herein, the transit-time effect causes the IGBT chip to oscillate. Because of the remaining plasma in the drift region, Plasma Extraction Transit Time (PETT) mechanisms can also be induced and make the device more likely to oscillate.
Expansion of the space charge area during the turn-off process, coupled with the transit-time effect, disperses the corresponding frequencies from single frequencies to a continuous frequency band, depending on the actual width of the space charge area. Parasitic components in IGBT modules such as stray bond wire inductance and junction capacitance of the free wheel diode and other parasitic capacitances and inductance in the module setup can build up a resonance circuit together with the IGBT chip. If the Eigen frequencies of such resonance circuit fall into the frequency band of the transient-time effect, TAO occurs. TAO behavior appears at the moment the gate voltage falls below the threshold voltage (i.e., moment of MOS channel turn off). At this moment, the DC-link voltage is partially established and at least part of the load current still flows. In IGBT modules, TAO can be observed on the measured signal of the gate voltage due to the electromagnetic coupling and can also be detected by placing an antenna near the emitter bond wire. Under extremely high current (e.g. >2× normal current) and high voltage, TAO in IGBT modules becomes strong enough to cause EMI disturbances.
Thus, there is a need to reduce TAO in power transistors.